Metal Doped Zeolite and Process for its Preparation

ABSTRACT

The present invention relates to a metal-doped or metal-exchanged zeolite, respectively, wherein the doping metal is present in the zeolite in the form of individual atoms i.e. as monomeric and/or dimeric species. The invention further relates to a process for the preparation of such a metal-exchanged zeolite. Said metal-doped zeolites are useful, in particular, for the reduction of nitrogen oxides.

The present invention relates to a metal-doped zeolite, wherein thecatalytically active doping metal is present isolated in the zeolite,i.e. as a monomeric species and/or dimeric species, and to a process forthe preparation of such zeolites and their use as catalyst material, inparticular for the purification of exhaust gases, quite particularly forthe reduction of nitrogen oxides.

Metal-doped zeolites are known from the state of the art and are widelyused as catalyst material for the purification of exhaust gases.

Because of the harmful effects of nitrogen oxides on the environment, itis important to further reduce these emissions. Clearly lower NOxemission limits for stationary and motor vehicle gases than arecustomary today are planned in the United States for the near future andare also being discussed in the European Union.

In order to be able to observe these limits, in the case of mobilecombustion engines (diesel engines) this can no longer be achieved bymeasures inside the engine, but only by an exhaust-gas post-treatment,for example with suitable catalysts.

The denitrification of combustion gases is also called DeNOx. Inautomobile engineering, selective catalytic reduction (SCR) is one ofthe most important DeNOx techniques. Hydrocarbons (HC—SCR) or ammonia(NH₃—SCR) or NH₃ precursors such as urea (Ad-Blue®) usually serve asreducing agents. Metal-exchanged zeolites (also called metal-dopedzeolites) have proved to be very active SCR catalysts that can be usedin a broad temperature range. They are mostly non-toxic and produce lessN₂O and SO₂ than the customary catalysts based on V₂O₅. In particulariron-doped zeolites represent good alternatives to the normally usedvanadium catalysts, because of their high activity and resistance tosulphur under hydrothermal conditions. Customary processes for dopingzeolites with metals comprise for example methods such as liquid ionexchange, solid-phase ion exchange, vapour-phase ion exchange,mechanical-chemical processes, impregnation processes and the so-calledextra-skeletal processes.

U.S. Pat. No. 5,171,553 discloses for example an ion-exchange process inan aqueous solution wherein silicon-rich zeolites with Si/Al ratios ofover 5 to approx. 50 are customarily used as support.

Problems result in particular when doping or introducing activecomponents such as e.g. iron, vanadium, cobalt and nickel into thezeolite, as different oxidation numbers of these catalytically activemetals occur next to each other and also the desired catalyticallyactive species is not always obtained, or the catalytically activespecies change into catalytically inactive species because of theparameters of the doping process (oxygen, temperature, moisture, etc.).

The doping of zeolites with iron by solid-state ion exchange is known(EP 0 955 080 B1), wherein a mixture of the desired zeolite, a metalcompound and an ammonium compound is sintered under a protectiveatmosphere, in particular a reductive protective atmosphere, with theresult that metal-containing, in particular iron-doped, catalysts withan increased long-term stability are obtained.

However, it has been shown that in practically all the known processesof the state of the art, cluster species of the catalytically activemetals, which are catalytically inactive or greatly reduce the catalyticactivity, form as a result of the metal exchange inside the zeolite. By“clusters” are meant polynuclear bridged or unbridged metal compoundswhich contain at least three identical or different metal atoms.Metal-exchanged zeolites in which no metal clusters were able to bedetected inside the zeolite skeleton are thus far unknown.

The object of the present invention was therefore to provide ametal-containing zeolite material which has an increased catalyticactivity compared with the metal-containing zeolite materials known thusfar.

This object is achieved by a metal-exchanged zeolite, wherein thereplacement metal is present in the zeolite as isolated individual atomsor cations (monomeric species) and/or as dimeric species. By“metal-exchanged” is meant that a metal or metal cation is replaced by acation, typically H⁺, Na⁺, NH₄ ⁻ etc., present in the zeolite or zeoliteskeleton.

The metal-exchanged zeolite according to the invention (here alsosynonymously called metal-doped zeolite) is free from catalyticallyinactive or catalytically less active metal clusters, with the resultthat only monomeric (isolated species in the form of individual metalatoms or metal cations) or dimeric catalytically highly-active metalspecies are present in the pore structure i.e. more precisely in thezeolite skeleton. The metal species can in other words also be called“skeleton metal species”. It is important that the term “monomeric ordimeric species” as used in the invention does not include so-called“ship-in-a-bottle” zeolitic host-guest complexes, such as have beendescribed e.g. in U.S. Pat. No. 5,603,914 or in S. Chavan et al. J.Catalysis 192, pp. 286-295 (2000) for cobalt-cyclopentadienyl, ordimeric copper acetate, incorporated in the zeolite pores.

The dimeric metal species in the zeolite according to the invention canbe present either bridged (e.g. bridged via O or OH atoms) or unbridged,thus have a metal-metal bond.

Much smaller quantities can thereby be used during metal doping or metalreplacement (often also called metal loading). Furthermore, the activityand selectivity of the zeolite according to the invention issignificantly increased compared with the known zeolites of the state ofthe art. It was found that generally, compared with the zeolites of thestate of the art doped or exchanged with the same metal in which, asexplained above, mostly metal clusters are present in the zeolite, themetal-exchanged zeolites according to the invention show an increase inactivity of approx. 30% for each metal during the reduction from NO toN₂. Inactive metal clusters also reduce the pore volume and impede thegas diffusion or lead to undesired secondary reactions, which can alsobe advantageously prevented by the zeolites according to the invention.

By “zeolite” is meant, within the framework of the present inventionaccording to the definition of the International MineralogicalAssociation (D. S. Coombs et al., Can. Mineralogist, 35, 1997, 1571), acrystalline substance from the group of the aluminium silicates with aspatial network structure of the general formula

M^(n+) _(n) [(AlO₂)_(x)(SiO₂)_(y)]i^(t)H₂O

which consist of SiO_(4/2) and AlO_(4/2) tetrahedra which are linked bycommon oxygen atoms to form a regular three-dimensional network. TheSi/Al=y/x ratio is always 1 according to the so-called “LöwensteinRule”, which states that two adjacent negatively-charged AlO₄ ⁻tetrahedral may not occur next to each other. Thus, although moreexchange spaces are available for metals with a low Si/Al ratio thezeolite becomes increasingly thermally unstable.

The zeolite structure contains voids and channels which arecharacteristic of each zeolite. The zeolites are divided into differentstructural types (see above) according to their topology. The zeoliteskeleton contains open voids in the form of channels and cages which arenormally occupied by water molecules and extra-skeletal cations whichcan be replaced. An aluminium atom attracts an excess negative chargewhich is compensated for by these cations. The interior of the poresystem is represented by the catalytically active surface. The morealuminium and the less silicon a zeolite contains, the denser is thenegative charge in its lattice and the more polar its inner surface. Thepore size and structure are determined, in addition to the parameters,during production (use or type of templates, pH, pressure, temperature,presence of seed crystals) by the Si/Al ratio which determines thegreatest part of the catalytic character of a zeolite. In the presentcase it is particularly preferred if the Si/Al ratio of a zeoliteaccording to the invention lies in the range from 10 to 20 (correspondsto a SiO₂/Al₂O₃ ratio of 20-40).

In liquid-exchange processes for the preparation of metal-doped or-exchanged zeolites there is a strong affinity for the replacement ofpolyvalent and heavy metal with lighter ones and in particular withhydrogen and/or NH₄.

In hydrated zeolites, dehydration takes place mostly at temperaturesbelow approximately 400° C. and is very largely reversible.

Because of the presence of 2- or 3-valent cations as tetrahedron centrein the zeolite skeleton the zeolite receives a negative charge in theform of so-called anion sites in whose vicinity the corresponding cationpositions are located. The negative charge is compensated for byincorporating cations, e.g. metal cations, into the pores of the zeolitematerial. A distinction between zeolites is drawn mainly according tothe geometry of the voids which are formed by the rigid network of theSiO₄/AlO₄ tetrahedra. The entrances to the voids are formed from 8, 10or 12 “rings” (narrow-, average- and wide-pored zeolites). Specificzeolites show a uniform structure (e.g. ZSM-5 with MFI topology) withlinear or zig-zag channels, while in others larger voids attachthemselves behind the pore openings, e.g. in the case of the Y and Azeolites with the topologies FAU and LTA. Generally, 10 and 12 “ring”zeolites are preferred according to the invention.

In principle, within the framework of the present invention, anyzeolite, in particular any 10 and 12 “ring” zeolite, can be used.Zeolites with the topologies AEL, BEA, CEA, EUO, FAO, FER, KFI, LTA,LTL, MAZ, MOR, MEL, MTW, LEV, OFF, TON and MFI are preferred accordingto the invention. Zeolites of the topological structures BEA, MFI, FER,MOR, MTW and TRI are quite particularly preferred.

According to the invention zeolite-like materials can likewise be used,such as are described for example in U.S. Pat. No. 5,250,282, to thefull disclosure content of which reference is made here. Further zeolitematerials preferred according to the invention are mesoporous zeolitematerials of silicates or aluminosilicates which are known under thename M41S and are described in detail in U.S. Pat. No. 5,098,684 andU.S. Pat. No. 5,102,643, to the full disclosure content of whichreference is likewise made here.

Further, so-called silico-aluminophosphates (SAPOs) which have formedfrom isomorphically exchanged aluminophosphates can be used according tothe invention.

It is preferred that the pore sizes of the zeolites used according tothe invention lie in the range from 0.4 to 1.5 nm which, also because ofthe more favourable steric ratios for monomeric or dimeric metalspecies, contributes advantageously to the formation of monomeric ordimeric metal species instead of metal clusters.

Typically the metal content or the degree of exchange of a zeolite isdecisively determined by the metal species present in the zeolite. Thezeolite can also be doped or exchanged with only one metal or withdifferent metals.

There are usually three different centres in zeolites described asso-called α-, β- and γ-positions, which define the position of theexchange spaces (also called “exchangeable positions or sites”). Allthree positions are available to reactants during the NH₃—SCR reaction,in particular when using MFI, BEA, FER, MOR, MTW and TRI zeolites.

The so-called α-type cations show the weakest bond to the zeoliteskeleton and are the last to be occupied in a liquid ion exchange. Thedegree of occupancy increases markedly from a degree of exchange ofaround 10% as the metal content increases and amounts to around 10 to50% in total at a degree of exchange of up to M/Al=0.5. Cations at thissite form very active redox catalysts.

On the other hand, the β-type cations which represent the most-occupiedposition and catalyze the HC—SCR reaction most effectively duringliquid-ion exchange, in particular with small degrees of exchange,display an average bonding strength to the zeolite skeleton. Thisposition is filled immediately after the γ-position and its degree ofoccupancy falls, from a degree of exchange of around 10%, as the metalcontent increases and amounts to around 50 to 90% for a degree ofexchange of up to M/Al=0.5. In the state of the art it is known thatfrom a degree of exchange of M/Al>0.56 typically only polynuclear metaloxides are still deposited.

The γ-type cations are those cations with the strongest bond to thezeolite skeleton and the most thermally stable. They are theleast-occupied position during liquid-ion exchange, but are filledfirst. Cations, in particular iron and cobalt, in these positions arehighly-active and are the catalytically most active cations.

The preferred metals (or metal cations) for the exchange or doping arecatalytically active metals such as Fe, Co, Ni, Ag, Co, V, Rh, Pd, Pt,Ir, and quite particularly preferred Fe, Co, Ni and Cu, which also formbridged dimeric species as are mainly present in particular at highdegrees of exchange in the case of the zeolite according to theinvention.

Overall, the quantity of metal calculated as corresponding metal oxideis 1 to 5 wt.-% relative to the weight of the metal-exchanged zeolite.In particular it is preferred that more than 50% of the exchangeablesites (i.e. α-, β- and γ-sites) are exchanged. Quite particularlypreferably, more than 70% of the exchangeable sites are exchanged.However, free sites should remain which are preferably Brønstedt acidcentres. This is because NO is strongly absorbed both at the exchangedmetal centres and also in ion-exchange positions or at Brønstedt centresof the zeolite skeleton. Moreover, NH₃ reacts for preference with thestrongly acid Brønstedt centres, the presence of which is thus veryimportant for a successful NH₃—SCR reaction. The presence of freeradical exchange spaces and/or Brønstedt acid centres and themetal-exchanged lattice places is thus quite particularly preferredaccording to the invention. Therefore, a degree of exchange of 70-90% ismost preferred. At a degree of exchange of more than 90%, a reduction inactivity is observed during the reduction of NO to N₂ and the SCR—NH₃reaction.

Because of the danger of hydrothermal deactivation of metal-exchangedzeolites, which is preceded by a dealuminization and migration of metalfrom the ion-exchange centres of the zeolite, it is preferred that thedoping metals if possible do not form a stable compound, as adealuminization is thereby promoted.

The object of the present invention is further achieved by a process forthe preparation of an above-described metal-exchanged doped zeolite,wherein firstly in a sealable reaction vessel an aqueous orwater-containing suspension of a zeolite is prepared, and wherein theprocess further comprises the steps of

-   -   a) increasing the pH of the suspension to a value in the range        from 8 to 10, preferably using NH₄OH (ammonia water) and setting        the oxygen level in the reaction vessel to a value of <10%,        preferably <5%    -   b) reducing the pH to a value in the range from 1.5 to 6    -   c) adding a metal salt and reacting over a period of 1 to 15        hours    -   d) filtering off and washing the metal-exchanged (doped)        zeolite.

The effect of controlling the reaction, in particular by increasing andreducing the pH (the so-called “pH control” of the process according tothe invention), is that a high degree of exchange of approx. 70-90% canbe achieved without metal clusters unsuitable for catalysis forming inthe metal-exchanged zeolite, something not previously achieved withprocesses known from the state of the art, in particular in the field ofliquid exchange.

The exchange rate and the extent of the exchange is further increased bythe pre-treatment, in particular with ammonia. After setting the oxygenlevel, the reaction mixture is stirred for approx. 1 to 60 minutes.

Surprisingly it was found that even if the pH is reduced to a value of1.5 to 6 in step b) of the process according to the invention,highly-exchanged zeolites (exchange limit approx. 90%) without metalclusters form, although thus far in the state of the art it has beenassumed and also found that, at low pHs, in most catalytically activemetals, polynuclear hydroxo- or oxo species form which are catalyticallyinactive.

By repeating the process according to the invention several times, thedegree of exchange or doping can be increased to 100% of theexchangeable sites in the zeolite although, as stated above, 90%represents the preferred upper limit. It is equally possible, albeit inless preferred embodiments, that different metals are used for exchangeor doping, varying the choice of metal salt in step c). Preferably, theabove-listed salts of the catalytically active metals, as of theirchlorides, sulphates, acetates, mixed ammonium-metal salts, nitrates andsoluble complex compounds are used.

The metal salt can be added both in solid form and also in the form of asolution, wherein aqueous solutions are preferred because they areeasier to work up, but there is nothing to prevent dissolved metal saltsalso being used in organic solvents or mixtures of aqueous or organicsolvents.

Typically, the resultant suspension contains 5 to 25 wt.-% zeolites inorder to guarantee a good thorough mixing by means of stirring, ashigher levels allow the mixture to solidify.

The pH is preferably increased by adding a strong base, quiteparticularly preferably in the form of ammonia water. Other suitablebases can also be used, of course, wherein bases such as NaOH or KOH areless advantageous because of their tendency to promote the formation ofpolynuclear hydroxo species with the metal salts.

The molar ratio of ammonia to zeolite is 0.01 to 0.1 in order to be ableto accurately set the pH level.

The suspension is typically acidified by adding a mineral acid such asHCl, H₂SO₄, HNO₃ etc. The acid, the anion of which is preferably alsothe anion of the corresponding metal salt which is then added, is used.HCl is less preferred however because of its high corrosivity duringwaste-water purification.

Quite particularly preferably the pH is thereby reduced to a range from1.5 to 3 which, as already stated unlike in the state of the art,surprisingly does not lead to polynuclear hydroxo-bridged metal-clusterspecies.

The acidification of the suspension is followed firstly by heating to atemperature in the range from 80 to 100° C. and then stirring forapprox. 2 hours. The increased temperature during ion exchangeadvantageously leads to the hydration sphere of the metal ions beingreduced and the exchange thus accelerated.

It is important, within the framework of the process according to theinvention, that the oxygen level in the reaction vessel during thereaction is <10 and quite preferably <5%, as here likewise the formationof polynuclear oxygen- or hydroxo-bridged metal species is suppressed.

The reaction time for the metal exchange is approx. 2 to 8 hours, quiteparticularly preferably 3 to 5 hours. The powder obtained after reactionand doping is washed filtered-off, optionally dried at above 100° C.,wherein the temperature during drying is not to exceed 250° C., asotherwise this leads to partial calcination.

A calcination can be carried out in a range from 400 to 600° C., quitepreferably under inert gas.

Usually, any metal salt mentioned above can be used, irrespective ofwhether the metal is present in a di-, tri- or quadrivalent oxidationstep, as the redox pairs usually balance during calcination. However,the divalent salts are quite particularly preferred in particular in thecase of iron salts, as these typically tend less towards theprecipitation of poorly soluble metal hydroxides and blocking of thezeolite pores. With higher-valency iron salts the chosen exchange timeshould therefore not be too long, because more hydroxides alsoprecipitate as a result.

The zeolite prepared by means of the process according to the inventionis typically used when purifying exhaust gases, in particular in thereduction of nitrogen oxides.

The invention is described in more detail below with reference toembodiment examples which are not, however, to be considered limiting.

General:

UV/VIS MIR diffuse reflection measurements were carried out on thezeolites obtained according to the invention using a Perkin Elmer UV/VISspectrometer with diffuse reflection and BaSO₄ as reference. Theabsorption intensities were evaluated according to theSchuster-Kubelka-Munk equation (often also called the Kubelka Munktheory).

EXAMPLE 1

Preparation of an Iron-Exchanged (or Doped) Zeolite

2 g NH₄—ZSM5 (alternatively H—ZSM5 or Na—ZSM5 was also used) wassuspended in an aqueous solution in a quantity of 10 to 15 wt.-%relative to the aqueous solution, and stirred at room temperature.Ammonia in the form of ammonia water was then added in a ratio of NH₃ tozeolite of 0.04, with the result that a pH of >9 was set. The pH was atmost 10.

The reaction vessel was then closed, flushed with inert gas and theatmosphere inside the vessel set to an oxygen level of <5%, followed bya wait of 20 min.

Sulphuric acid in the form of dilute sulphuric acid concentration (25vol. %) was then added, wherein the ratio of sulphuric acid to zeolitewas 0.05. The oxygen level was left at <5% and a pH of 4.0 set. Theacidified suspension was then heated to a temperature of 90° C. andthereafter immediately solid FeSO₄.7H₂O added in a weight ratio ofFeSO₄:7H₂O to zeolite of 0.2. The pH was 3 and the reaction was carriedout in the reaction vessel over 8 hours accompanied by stirring at anoxygen level of less than 5%.

The exchanged zeolite, which had a virtually white colour, was thenfiltered and washed three times with distilled water and dried at 150°C. Calcination took place under inert gas at 500° C. for 3 hours.

The resulting product contained 1.5 wt.-% Fe₂O₃ relative to the totalmass of zeolite.

UV/VIS Spectrum:

The thus-obtained iron-exchanged zeolite showed no bands in thewavelength range of 10-25,000 cm⁻¹ assigned to the polynuclear Feclusters (i.e. more than 3 Fe atoms) which are only slightly, if at all,active during the SCR reaction.

On the other hand, the iron-exchanged zeolite according to the inventionshows thick bands in the wavelength range of 25,000 to 30,000 cm⁻¹,which are allocated to iron-oxide dimers (Fe—O—Fe) which are highlyactive during the SCR reaction. Likewise, the zeolite obtained inexample 1 showed bands in the wavelength range between 30,000 and 50,000cm⁻¹, which can be allocated to monomers, iron species arranged in thezeolite lattice, or in particular monomeric FeOH species which arelikewise highly active for the SCR reaction.

A catalyst prepared by means of the iron zeolite according to theinvention obtained in example 1 thus does not have polynuclearcatalytically inactive iron clusters.

EXAMPLE 2

Preparation of a Cobalt-Exchanged Zeolite

The reaction took place as in example 1, except that in place of ironsulphate (FeSO₄.7H₂O), a corresponding quantity of Co(NO₃)₂ oralternatively Co(acac)₂((acac)=acetylacetonate) was used. Instead ofsulphuric acid, the corresponding quantity of 0.01 M HNO₃ was used.

UV/VIS: 38-45,000 cm-¹ (m) (SCR active monomers and dimeric Co-centres),no bands at 10 to 15,000 cm⁻¹ (cluster species).

EXAMPLE 3

Preparation of a Copper-Exchanged Zeolite

Synthesis took place as in example 1, except that a correspondingquantity of copper-acetylacetonate solution was used as metal salt andagain 0.01 M HNO₃ as acid.

UV/VIS: 37-45,500 cm⁻¹ (m) (SCR active monomers and dimeric co-centres),no bands at 10 to 15,000 cm⁻¹ (cluster species).

EXAMPLE 4

Preparation of a Silver-Exchanged Zeolite

Synthesis took place as in example 1, except that AgNO₃ was used asmetal salt and 0.01 M HNO₃ as acid.

EXAMPLE 5

Preparation of a Nickel-Exchanged Zeolite

Synthesis took place as in example 1, except that Ni(NO₂)₂ was used asmetal salt and 0.01 M HNO₃ as acid.

EXAMPLE 6

The catalyst obtained in example 1 was tested during the reduction of NOto N₂.

The catalyst obtained according to the invention was hydrothermally agedat 800° C. for 12 hours in an atmosphere with 10% water vapour, thenpressed into shaped bodies and sieved to a size of 0.4 to 0.8 mm.

Likewise, a comparison catalyst which was obtained according to theexample of EP 955 080 B1 was hydrothermally aged in the same manner.This comparison catalyst was obtained by solid-state ion exchange.

The exhaust-gas composition for the comparison test was:

NO: 500 ppm

NH₃: 500 ppm

H₂O: 50 vol. %

SV (space velocity): 80,000

Remainder N₂.

The test was carried out under customary test conditions.

TABLE 1 Conversion of NO to N₂ Fe zeolite Comparison Temperature example1 Fe zeolite 200° C. 35% 24% 350° C. 95% 88% 500° C. 92% 88%

As can be seen from Table 1, the conversion of NO to N₂ using a catalystobtained according to the invention is clearly higher at 350° C. thanwhen using a comparison catalyst from the state of the art. Thetemperature of 350° C. is approximately the optimum temperature for theabove-named DeNO_(x) processes.

The increase in activity at 350° C. was thus approx. 58%.

1. Metal-exchanged zeolite, wherein the replacement metal is present inthe zeolite as a monomeric and/or as a dimeric species.
 2. Zeoliteaccording to claim 1, characterized in that the zeolite is selected fromthe zeolites of the structural types AEL, BEA, CHA, EUO, FAO, FER, KFI,LTA, LTL, MAZ, MOR, MEL, MTW, LEV, OFF, TON and MFI.
 3. Zeoliteaccording to claim 2, characterized in that the zeolite is selected fromthe zeolites with the structural types BEA, MFI, FER, MOR, MTW, ERI. 4.Zeolite according claim 2, characterized in that the pore size of thezeolites is 0.4 to 1.5 nm.
 5. Zeolite according to claim 4,characterized in that the replacement metal is catalytically active. 6.Zeolite according to claim 5, characterized in that the replacementmetal is selected from the group consisting of Fe, Co, Ni, Ag, Cu, V,Rh, Pd, Pt, Ir.
 7. Zeolite according to claim 6, characterized in thatthe replacement metal is selected from the group consisting of Fe, Co,Ni, Cu.
 8. Zeolite according to claim 6, characterized in that thereplacement metal is present in a quantity of 1 to 5 wt.-% calculated asmetal oxide relative to the total weight of the zeolite.
 9. Zeoliteaccording to claim 8, characterized in that more than 50% of theexchangeable sites of the zeolite skeleton are occupied by thereplacement metal.
 10. Process for the preparation of a metal-exchangedzeolite according to one of the previous claims, wherein firstly anaqueous suspension of a zeolite is prepared in a sealable reactionvessel, further comprising the steps of a) increasing the pH of thesuspension to a value in the range from 8 to 10, and setting the oxygenlevel of the reaction vessel to a value of <10%, b) reducing the pH to avalue in the range from 1.5 to 6, c) adding a metal salt and reactingover a period of 1 to 15 hours, d) filtering off and washing themetal-exchanged zeolite.
 11. Process according to claim 10,characterized in that the suspension contains 5 to 25 wt.-% zeolite. 12.Process according to claim 11, characterized in that the increase in pHin step a) takes place by adding ammonia in the form of ammonia water.13. Process according to claim 12, characterized in that the molar ratioof ammonia to zeolite has a value of 0.01 to 0.1.
 14. Process accordingto claim 13, characterized in that the pH is reduced in step b) byadding a mineral acid.
 15. Process according to claim 14, characterizedin that the pH is set to a value in the range from 1.5 to
 3. 16. Processaccording to claim 14, characterized in that, after reducing the pH, thesuspension is heated to a temperature in the range from 80 to 100° C.17. Process according to claim 16, characterized in that, during theexchange reaction in step c), the oxygen level in the reaction vessel isset to <5%.
 18. Process according to claim 17, characterized in that thezeolite obtained after step d) is dried at a temperature of >100° C. 19.Process according to claim 17, characterized in that the process iscarried out several times.
 20. Process according to claim 18,characterized in that the dried zeolite is calcined at a temperature of400 to 600° C.
 21. Process according to claim 20, characterized in thatthe calcination takes place under inert gas.
 22. Metal-exchanged zeolitein which the metal is present as a monomeric and/or as dimeric species,obtainable by the process according to claim 10.